This invention relates to ferroelectric liquid crystal cells, and in particular to a method of obtaining a preferred alignment of the liquid crystal molecules within such cells.
The conventional molecular alignment required for operation of a ferroelectric liquid crystal cell is one in which the smectic layers are formed in planes orthogonal to the major surfaces of the cell, such arrangements sometimes being termed `bookshelf geometry`.
In suitable circumstances bookshelf geometry can be obtained for a material exhibiting the phase sequence EQU I-N*-S.sub.A -S.sub.C *
by slow cooling of a cell that has rubbed major surfaces that promote planar alignment of the molecules in the nematic phase. (At least some of the individual molecules of the liquid crystal medium have to have a chiral centre for the medium to be ferroelectric in the tilted phase. The presence of a chiral component in a nematic phase normally induces a regular helical structure and the phase is termed cholesteric. A helical structure would interfere with the requisite alignment in the smectic phases and so is effectively eliminated by the use of suitable compensating chiral constituents of opposite handedness to produce a phase, commonly designated N*, in which any residual cholesteric pitch is large compared with cell thickness. As the material cools into the N* phase the molecules assume planar alignment, with the molecular director lying in the rubbing direction, and this alignment is preserved as the material enters the S.sub.A phase. The smectic layers that are then produced lie in planes extending orthogonally with respect to the planes of the major surfaces of the cell, and on further cooling into the S.sub.C * phase it is intended that the alignment of these smectic layers shall be substantially preserved while the directors of the molecules are rotated through a small angle to produce a tilted smectic phase.
In practice uniform alignment of the smectic A phase layers can be achieved quite easily when the major surfaces of the liquid crystal layer are confined by rubbed polymer film, but excellent alignment in the S.sub.A phase is found no guarantee of achieving satisfactory alignment in the S.sub.C * phase. Particularly in the case of thin cells (having a liquid crystal layer thickness of about 2 microns), such as are typically employed for high speed switching, the density of alignment defects can be extremely high in the S.sub.C * phase. Such defects adversely affect both the persistence of switching and the contrast ratio.
The wide occurrence of these defects is believed to be attributable in large part to the fact that the mechanism used to align the molecular director in the N* phase is liable to continue to exert an influence on molecular alignment when the liquid crystal layer is in the S.sub.C * phase. In the S.sub.C * phase, however, the rubbing direction is still normal to the planes of the smectic layers, and hence is not a direction appropriate for the director in a tilted smectic phase. Strain is therefore associated with this form of alignment. If both major surfaces of the cell are similarly aligned the strain concentration is seen to be increased as the liquid crystal layer thickness is reduced. It is possible for a situation to arise when it is energetically more favourable for the smectic layers to reorient than for the director to accommodate all the strain within the layers. It is believed that this may be a major factor contributing to the break-up of the layers in thin cells. Another contributing factor may be small misalignments of rubbing direction between the two major surfaces confining the liquid crystal layer.
Investigations have been made to see if the problem of the break-up of the layers is alleviated by constructing cells where only one of the confining surfaces is provided by a rubbed polymer layer instead of both surfaces. We have found that this can be the case with cells in which one confining surface is provided by a rubbed polymer covered transparent electroded glass substrate, while the other confining surface is provided by a similar substrate without the rubbed polymer layer. However, it has been found that the quality of the result is critically dependent upon the absence of blemishes in the surface of the second substrate. Preparation of satisfactory cells was found to be difficult, time consuming, and unpredictable. Much better results have been obtained by adopting the teachings of the present invention.